text: Photochemical conversion occurs due to interactions between the chromophoric unit and residues in its vicinity. Glu-212 functions as a base that removes a proton from His-62 aiding in the cleavage of the His-62-Nα-Cα bond. Replacing Glu-212 with glutamine prevents photoconversion.  At low pH, the yield of Eos involved in photoconversion is greatly increased as the fraction of molecules in the protonated form increases. The action spectrum for photoconversion is closely related to the action spectrum for Eos's protonated form.  These observations suggest that the neutral form of the green chromophore, including a protonated Tyr-63 side chain, is the gateway structure for photoconversion.  Proton ejection from the Tyr-63 phenyl side chain is an important event in the conversion mechanism where a proton is transferred from the  His-62 imidazole, which is hydrogen-bonded to the Phe-61 carbonyl.  The extra proton causes His-62 to donate a proton to the Phe-61 carbonyl forming a leaving group out of the peptide bond between His and Phe in the elimination reaction.  The His-62 side chain is protonated during photoexcitation and assists the reaction by donating a proton to the Phe-61 carbonyl in the leaving group.  After the backbone is cleaved, the hydrogen bond between His-62 and Phe-61 is reformed.  When His-62 is replaced with other amino acids, EosFP loses its ability to photoconvert, providing evidence that His-62 is a necessary component of the photoconversion mechanism. The internal charge distribution of the green chromophore is altered during photo excitation to assist in the elimination reaction.